Some injection molded articles, for example plastic preforms of the variety that are for blow molding into beverage bottles, require extended cooling periods to solidify into substantially defect-free molded articles. To the extent that the cooling of the molded article can be effected outside of the injection mold by one or more post-mold devices then the productivity of the injection mold may be increased (i.e. lower cycle time). A variety of such post-mold devices, and related methods, are known and have proven effective at the optimization of the injection molding machine cycle time.
In a typical injection molding system, such as the system 10 depicted with reference to FIG. 1, and as generally described in commonly assigned U.S. Pat. No. 6,171,541 (Inventor: NETER, Witold, et al.; Published: 9 Jan. 2001), just-molded, and hence partially cooled, molded articles 2 are ejected from the mold half 8, when the mold halves 8, 9 are spaced apart, and into holders 50 (i.e. commonly known as a cooling holder, a take-off holder, or a cooling pipe, amongst others). The holders 50 are arranged on a post-mold device 15 (i.e. commonly known as an end-of-arm-tool, carrier plate assembly, removal device, post-cooling apparatus, amongst others), the post-mold device 15 configured to cyclically position the holders 50, arranged on a supporting plate 16, between an in-mold position between the mold halves 8, 9, to receive the molded articles 2, and an out-board position, as depicted, to allow the mold halves 8, 9 to close and begin another molding cycle. The construction and operation of the post-mold device 15, including those having multiple-positions, is generally described in commonly assigned U.S. Pat. RE33,237 (Inventor: DEFLER, Frank; Published: 19 Jun. 1990). Preferably, the molded articles 2 are held in the holders 50 until the molded articles 2 have cooled sufficiently that they may be ejected without risk of further deformation. The injection molding machine includes a controller 30, such as that described in commonly assigned U.S. Pat. No. 6,275,741 (Inventor: CHOI, Christopher; Published: 14 Aug. 2001), for controlling machine-control functions.
The cooling of the molded articles 2 may be assisted by the use of pins 14 for expelling a cooling fluid onto an inner portion of the molded articles 2, as shown with further reference to FIG. 2B. The pins 14 are arranged on another post-mold device 12 (i.e. commonly known as a COOLJET, a trademark of Husky Injection Molding Systems Ltd.), the post-mold device 12 arranged to be cyclically positioned between a cooling position, with the pins 14 positioned adjacent the portion of the molded articles 2, and an out-board position, as depicted. It is also known to use the molded article post-mold device 12 to extract the molded articles 2 from the holders 50 for a re-handling thereof, for instance, to a conveyor.
A portion of the post-mold device 15 depicting a holder 50 arranged on the supporting plate 16 is shown with reference to FIGS. 2A and 2B. The holder 50 is configured in accordance with the general teachings of commonly assigned U.S. Pat. No. 4,729,732 (Inventor: SCHAD, et al.; Published: 4 Mar. 1988). In particular, the holder 50 includes a tapered surface 52 defining a cavity for receiving a portion of the molded article 2, the surface 52 being smaller than the heated molded article. The holder includes a cooling structure operative to shrink the molded article, upon cooling, with the molded article sliding inside the cavity to fit snugly therein. The holder 50 further includes a suction structure adjacent a closed end of the cavity for maintaining the molded article in the holder 50.
As shown with reference to FIG. 2B, the cooling of the molded articles 2 may be assisted by the use of a coolant dispersion device 19 of a post-mold device 13 for dispersion of a coolant, such as cool air, around an exposed outer portion of the molded article; as generally described in commonly assigned U.S. Pat. No. 6,802,705 (Inventor: BRAND, Tiemo, et al.; Published: 12 Oct. 2004).
FIG. 2A depicts an initial position of the molded article 2 in the holder 50 immediately after having been received from the mold.
FIG. 2B depicts a completely seated position of the molded article 2 in the holder 50 after cooling, and related shrinkage, of the molded article 2.
The holder 50 comprises a holder 60 and an insert 70. The insert 70 is arranged in the holder 60 to provide the closed end of the cavity. The suction structure comprises a pressure channel 54 that extends through the insert 70, the channel 54 is connectable to an air pressure source 18, provided in a plate 16 of the post-mold device 15, via a pressure channel 18′ configured in the holder 60. Likewise, the cooling structure comprises a coolant channel 62 configured around the holder 60, and enclosed by a holder sleeve 64, the coolant channel 62 connectable to a coolant source 17, provided in the plate 16, via a coolant channel 17′ in the plate 16. The holder 60 and the insert 70 are held on the plate 16 by a fastener 72.
The coolant source 17 in the plate 16 is typically directly connected to a plant-wide coolant source. Typical plant-wide coolant sources include a chiller or a cooling tower to remove the heat added to the coolant from the molded article in the holder. Presently, faced with the problem of improving the efficiency of a molding cycle the common general knowledge in the molding art is to remove heat from the molded article holder as quickly as possible. The coolant, typically water, is preferably cooled to a temperature in the range of 6-10° C. In some high humidity molding environments the coolant may be kept warmer to avoid unwanted water condensation on the holder 50.
As can be seen with reference to FIGS. 2A and 2B, a first portion of the molded article 2′ that is received in the cooled holder 50 will be cooled, by the holder 50, at a first rate while a second portion of the molded article 2″ that is outside of the holder 50 will be cooled at a second rate. Under certain circumstances the second portion of the molded article 2″ can take longer to cool than the first portion of the molded article 2′. The relative cooling between the first and second portions of the molded article 2′, 2″ may be affected by one or more variables such as the distribution of plastic in the molded article 2, the thermal profile of the molded article when ejected from the mold 8, 9, the relative first and second rates of cooling, amongst others. Whenever the time required for post-mold cooling the second portion of the molded article 2″ is the limiting factor there is the risk that the first portion of the molded article 2′ may become over-cooled. An over-cooled first portion of the molded article 2′ is prone to deform.
Problems associated with cooling molded articles in the holder 50 may include localized sink marks and ovality.
With the relatively long molding cycle-times of the past it was generally possible to adjust the geometry of the cavity in the holder 50 to address the known defects. For instance, ovality defects may be addressed by adjusting the cavity in the holder 50 to be slightly smaller.
With increasingly aggressive molding cycle-time it is not always possible to address the defects by simple adjustment of the cavity geometry in the holder as adjusting the geometry for one defect may have the effect of making the another defect more prominent.